1. Field of the Invention
The present invention relates to a motor control device for controlling a permanent magnet reluctance motor.
2. Description of the Related Art
It has hitherto been common practice to implement field-weakening control during constant output operation in permanent magnet and reluctance motors when inverter-driven for use in electric trains, electric motor vehicles and similar applications, the aim being to ensure that the motor terminal voltage is lower than the maximum voltage which the inverter is capable of outputting.
FIG. 1 illustrates an example of a conventional motor control device with field-weakening control of this kind. This control device has a dq axes current command setting unit 11, a d-axis current command correction unit 14, a d-axis current control unit 16, a q-axis current control unit 17, a voltage vector length calculation unit 18, a voltage vector length restriction unit 19, a terminal voltage uniformity control unit 20, and a dq three-phase transformation unit 21. It should be added that in this example of the prior art it is assumed that the permanent magnet reluctance motor is driven under vector control.
The dq axes current command setting unit 11 inputs the torque command Tref, determining the d-axis current command Idref and q-axis current command Iqref required in order to output this torque. The d-axis current command Idref is corrected by the d-axis current command correction unit 14 by adding the d-axis current command correction value xcex94Idref from the terminal voltage uniformity control unit 20, and this is input to the d-axis current control unit 16. The q-axis current command Iqref is input to the q-axis current control unit 17.
The d-axis current control unit 16 inputs the d-axis current command Idref fed from the d-axis current command correction unit 14 and the d-axis current feedback value Id, and generates a d-axis voltage command Vd in such a manner that the d-axis current feedback value Id tracks the d-axis current command Idref. This is input to the dq three-phase transformation unit 21. Similarly, the q-axis current control unit 17 inputs the q-axis current command Iqref fed from the q-axis current command correction unit 11 and the q-axis current feedback value Iq, and generates a q-axis voltage command Vq in such a manner that the q-axis current feedback value Iq tracks the q-axis current command Iqref. This is input to the dq three-phase transformation unit 21. The dq three-phase transformation unit 21 generates the three-phase voltage commands Vu, Vv and Vw on the basis of the d-axis voltage command Vd, the q-axis voltage command Vq and the motor rotor potential xcex8 r, controlling the motor by way of a voltage transformer not illustrated in the drawing in order to achieve this.
The voltage vector length calculation unit 18 inputs the dq axes voltage commands Vd and Vq and calculates the voltage vector length (absolute voltage value) Vl. The voltage vector length restriction unit 19 inputs the resultant voltage vector length Vl and the inverter input direct-current voltage Vdc, and determines the restricted voltage vector restriction length Vllim. The terminal voltage uniformity control unit 20 calculates the current command correction value xcex94Idref on the basis of the voltage vector length Vl and the voltage vector restriction length Vllim, and inputs it to the d-axis current command correction unit 14 as already mentioned.
In the drawing, the circuit elements represented by the codes 18, 19, 20 and 14 are for the purpose of field-weakening control. Field-weakening control of a permanent magnet motor generally involves running an armature current or minus d-axis current, so to speak, so that the magnetic flux of the permanent magnet and the magnetic flux created by the current flowing to the motor armature are in opposite directions. In a reluctance motor, on the other hand, where there is a large inductance value axis (q-axis) and a small inductance value axis (d-axis), it is normal to achieve field-weakening control by reducing the more effective q-axis current.
However, when it is sought to implement field-weakening control in a motor which generates a combination of reluctance torque and torque resulting from a permanent magnet, this can prove ineffective depending on the magnitude of the current amplitude rendered variable by the torque which it is desired to output if the current which is allowed to flow for this purpose is fixed on either the d-axis or the q-axis. The result is that it becomes impossible to control the motor terminal voltage below the maximum inverter output voltage, and control becomes unstable. A similar phenomenon can be produced also in a so-called embedded-type permanent magnet motor which outputs combined permanent magnet and reluctance torque by virtue of the fact that the permanent magnet is embedded within the rotor core.
Accordingly, one object of the present invention is to provide a novel motor control device wherein it is possible to implement field-weakening control in a stable and effective manner whatever torque is output, thus solving the abovementioned problem.
With a view to achieving the abovementioned object, the present invention is a motor control device for controlling a permanent magnet reluctance motor which generates torque corresponding to the combined value of the torque resulting from the permanent magnet and the reluctance torque, having a means of correction which serves to correct the current command value in such a manner as to prevent the motor terminal voltage from exceeding the maximum inverter output voltage, and a means of variation which serves to render the angle between the current command value from this means of correction and the motor rotor variable in accordance with the magnitude of the given torque command.